Method of resetting spin valve heads in a magnetic disk drive

ABSTRACT

A process for resetting or initially establishing the magnetic orientation of one or more spin valves in a magnetoresistive read head with improved robustness. The spin valve includes subcomponents such as an antiferromagnetic layer, a ferromagnetic pinned layer, a conductive layer, a free layer, and a hard bias layer. A first external magnetic field is first applied to the spin valve sensor, this field having a first orientation relative to the spin valve sensor. During application of the first external magnetic field, a pulse of electrical current is directed through the spin valve sensor in a first direction, preferably parallel to the magnetic orientation of the external field. The current waveform brings the antiferromagnetic layer of the spin valve past its blocking temperature, freeing its magnetic orientation. The first external field exerts a robust bias upon the antiferromagnetic layer in the desired direction. Depending upon its flow direction, the current pulse may contribute an internal magnetic field that cooperates in magnetically biasing the antiferromagnetic layer as desired. After the current pulse, the antiferromagnetic layer cools below its blocking temperature, retaining the desired biasing. After the first external field is removed, and a second external magnetic field may be applied to the spin valve sensor for a predetermined time. The second external field is oriented to magnetically direct the hard bias layer of the sensor, thereby establishing the quiescent magnetization of the sensor&#39;s ferromagnetic free layer.

This is a divisional application of application Ser. No. 08/854,978filed May 13, 1997, now U.S. Pat. No. 6,118,622.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to sensors for reading magnetic fluxtransitions from magnetic media such as disks and tape. Moreparticularly, the invention concerns a technique for resetting themagnetic orientation of one or more spin valves in a magnetoresistiveread head.

2. Description of the Related Art

A magnetoresistive (“MR”) sensor detects magnetic field signals bymeasuring changes in the resistance of an MR element, fabricated of amagnetic material. Resistance of the MR element changes as a function ofthe strength and direction of magnetic flux being sensed by the element.Conventional MR sensors operate on the basis of the anisotropicmagnetoresistive (“AMR”) effect, in which a component of the element'sresistance varies as the square of the cosine of the angle between themagnetization in the element and the direction of sense or bias currentflow through the element.

MR sensors are useful in magnetic recording systems where recorded datais read from a magnetic medium. In particular, the external magneticfield from the recorded magnetic medium (the signal field) causes achange in the direction of the magnetization of an MR head. This in turncauses a change in electrical resistance in the MR read head and acorresponding change in the sensed current or voltage.

A variety of magnetic multilayered structures demonstrate asignificantly higher MR coefficient than an AMR sensor. This effect isknown as the giant magnetoresistive (“GMR”) effect. The essentialfeatures of these structures include at least two ferromagnetic metallayers separated by a nonferromagnetic metal layer. This GMR effect hasbeen found in a variety of systems, such as iron-chromium (FeCr) andcobalt-copper (CoCu) multilayers exhibiting strong antiferromagneticcoupling of the ferromagnetic layers. The GMR effect is also found inessentially uncoupled layered structures in which the magnetizationorientation in one of the two ferromagnetic layers is fixed or pinned.The physical origin is the same in all types of GMR structures: theapplication of an external magnetic field causes a variation in therelative orientation of the magnetizations of neighboring ferromagneticlayers. This in turn causes a change in the spin-dependent scattering ofconduction electrons and thus the electrical resistance of thestructure. The resistance of the structure thus changes as the relativealignment of the magnetizations of the ferromagnetic layers changes.

One specific application of GMR is the spin valve sensor. Spin valvesensors include a nonmagnetic conductive layer called a “spacer” layer,sandwiched between “pinned” and “free” ferromagnetic layers. Themagnetization of the pinned layer is pinned 90° to the quiescentmagnetization of the free layer. Unlike the pinned layer, the free layerhas a magnetic moment that freely responds to external magnetic fields,including those from a magnetic disk.

A spin valve sensor may be used to read data by directing a sensecurrent through the free, spacer, and pinned layers of the sensor. Theresistance of the spin valve sensor changes in proportion to rotation ofthe magnetic free layer (which moves freely) relative to the pinnedlayer (which is fixed in place). Such changes in resistance are detectedand ultimately processed as playback signals.

In a typical spin valve MR sensor, the free and pinned layers have equalthicknesses, but the spacer layer is one half as thick as either of thefree or pinned layers. An exemplary thickness of each of the free andpinned layers is 50 Å and an exemplary thickness of the spacer layer is25 Å.

As mentioned above, the magnetization of the pinned layer is pinned 90°to the magnetization of the free layer. Pinning may be achieved bydepositing the ferromagnetic layer to be pinned onto anantiferromagnetic layer to create an interfacial exchange couplingbetween the two layers. The antiferromagnetic layer may be constructedfrom a group of materials which include FeMn, NiMn, and NiO.

The spin structure of the antiferromagnetic layer can be aligned along adesired direction (in the plane of the layer) by heating beyond the“blocking” temperature of the antiferromagnetic layer and cooling in thepresence of a magnetic field. The blocking temperature is thetemperature at which the magnetic spins within a material lose theirorientation. In other words, a material's blocking temperature isreached when exchange anisotropy vanishes because the local anisotropyof the antiferromagnetic layer, which decreases with temperature, hasbecome too small to anchor the antiferromagnetic spins to thecrystallographic lattice. The blocking temperatures of manyantiferromagnetic materials ranges from about 160° to 200° C. Thus, whenthe blocking temperature of the antiferromagnetic material is exceeded,the spins of the antiferromagnetic layer lose their orientation causingthe first ferromagnetic layer to no longer be pinned.

Unlike the pinned layer, the free layer has a magnetic moment thatfreely responds to external magnetic fields, including those from amagnetic disk. The thickness of the spacer layer is chosen to be lessthan the mean free path of conduction electrons through the sensor. Withthis arrangement, a portion of the conduction electrons are scattered bythe interfaces of the spacer layer with the pinned and free layers. Whenthe magnetizations of the pinned and free layers are parallel withrespect to one another, scattering is minimal; when the magnetizationsof the pinned and free layers are antiparallel, scattering is maximized.Due to changes in scattering, the resistance of the spin valve sensorchanges in proportion to the cosine of the angle between themagnetizations of the pinned and free layers.

A number of U.S. patents disclose spin valve sensors. One patent, forexample, shows a spin valve sensor in which at least one of theferromagnetic layers is Co an alloy thereof, where the magnetizations ofthe two ferromagnetic layers are maintained substantially perpendicularto each other at zero externally applied magnetic field by exchangecoupling of the pinned ferromagnetic layer to an antiferromagneticlayer. See, e.g., U.S. Pat. No. 5,159,513, assigned to InternationalBusiness Machines Corp. Another patent discloses a basic spin valvesensor where the free layer is a continuous film having a central activeregion and end regions. The end regions of the free layer are exchangebiased by exchange coupling to one type of antiferromagnetic material,and the pinned layer by exchange coupling to a different type ofantiferromagnetic material. See, e.g., U.S. Pat. No. 5,206,590.

A read head employing a spin valve sensor, called a “spin valve readhead”, may be combined with an inductive write head to form a “combined”head. The combined head may have the structure of either a merged head,or a piggyback head. In a merged head a single layer serves as a shieldfor the read head and as a first pole piece for the write head. Apiggyback head has a separate layer which serves as the first pole piecefor the write head. In a magnetic disk drive an air bearing surface(“ABS”) of a combined head is supported adjacent a rotating disk towrite information on or read information from the disk. Information iswritten to the rotating disk by magnetic fields which fringe across agap between the first and second pole pieces of the write head.

To read data, a sense current is directed through the free, spacer, andpinned layers of the sensor. The resistance of the spin valve sensorchanges in proportion to relative rotation of the magnetic moments ofthe free and pinned layers. Such changes in resistance are detected andultimately processed as playback signals.

Known spin valve sensors provide a number of benefits, most notablytheir significantly higher MR coefficient in comparison to AMR sensors.However, spin valves are sensitive to heating, which can disorient themagnetic spins in both antiferromagnetic and ferromagnetic films of thespin valve. This occurs whenever the heat source exceeds the blockingtemperature of the antiferromagnetic films.

The chief sources of heat are electrostatic discharge and electrostaticoverstress. Electrostatic discharge often ruins a sensor completely,whereas electrostatic overstress usually reduces the sensor'sefficiency. These blocking temperatures can be reached by certainthermal effects during operation of the disk drive, such as an increasein the ambient temperature inside the drive, heating of the spin valvesensor due to the bias current, and rapid heating of the spin valvesensor due to the head carrier contacting asperities on the disk. Inaddition, magnetic disk drives are especially vulnerable toelectrostatic discharge during the manufacturing process, such as duringfabrication and assembly. If any of these thermal effects cause the spinvalve sensor to exceed the antiferromagnet's blocking temperature, themagnetization of the pinned layer will no longer be pinned in thedesired direction. This changes the spin valve sensor's response to anexternally applied magnetic field, resulting in errors in data read fromthe disk.

A number of precautions are taken to avoid the dangers of heat-inducedmagnetic disorientation. For example, during the manufacturing processtechnicians can electrically ground themselves and their workpieces.Nonetheless, damage to spin valve sensors still occurs under somecircumstances. Electric over stress can change pinned layermagnetization orientation. This is due to the fact that the heating bythe current raises the temperature of the head near to the blockingtemperature. Since the exchange field drops to near zero around theblocking temperature, antiferromagnetic layer spins around a neighboringferromagnetic layer (pinned layer) magnetization will assume thedirection of the field generated by the current. However, the field fromthe sense current is only of limited value (around only about 20 Oe).Therefore, if the pinned layer has coercivity in addition to theexchange field, and if the coercivity value is larger than the fieldfrom the current, then the field from the current will not be able toproperly orient the pinned layer magnetization. Coercivity, in contrastto the exchange field, does not drop so strongly with temperature; as aresult, coercivity can be fairly high, even at elevated temperatures. Inaddition, since the field from the current is non-uniform over theactive area of the sensor it does not set the magnetization of thepinned layer over the entire pinned layer. As a result of these factors,electric overstress can severely diminish or disable the functionalityof a spin value sensor.

SUMMARY OF THE INVENTION

Broadly, the present invention concerns a process for resetting themagnetic orientation of one or more spin valves in one or moremagnetoresistive read heads. Each spin valve includes subcomponents suchas an antiferromagnetic layer, a ferromagnetic pinned layer, aconductive layer, a free layer, and a hard bias layer.

For each spin valve, a first external magnetic field is first applied tothe spin valve sensor. This field has a first orientation relative tothe spin valve sensor. During application of the first external magneticfield, a waveform of electrical current is directed through the spinvalve sensor. The electrical current brings the antiferromagnetic layerof the spin valve past its blocking temperature, freeing its magneticorientation. The external field exerts a robust bias upon theantiferromagnetic layer in the desired direction.

The invention may be applied, for example, to properly orient themagnetization of the pinned layer just before the heads are put into thedisk drive enclosure. The actuator assembly (“head stack”) is emersed ina large external field. The external field is perpendicular to the airbearing surface of the head. With external field on, the heads arepulsed with the electric current for very brief duration, e.g., 60 to120 ns. The current pulse raises the temperature near to the blockingtemperature, and external field freezes the pinned layer magnetizationas the heads cool below the blocking temperature.

After the current pulse, the antiferromagnetic layers cool below theirblocking temperatures, and retain the desired biasing. The externalfield is removed, and a second external magnetic field is applied to thespin valve sensor for a predetermined time. The second external field isoriented to magnetically direct the sensor's hard bias layers, therebyestablishing the quiescent magnetization of the sensor's ferromagneticfree layer.

The invention affords its users with a number of distinct advantages.Chiefly, the invention provides a more robust technique for resetting aspin valve sensor that has suffered magnetic disorientation due toelectrostatic trauma. The external magnetic field provides a stronger,more uniform biasing field than the electrical current of the pulsealone. Consequently, after a spin valve is reset according to theinvention it reads data with greater sensitivity and accuracy. Theinvention also provides a number of other benefits, as discussed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The nature, objects, and advantages of the invention will become moreapparent to those skilled in the art after considering the followingdetailed description in connection with the accompanying drawings, inwhich like reference numerals designate like parts throughout, wherein:

FIG. 1 is a cross-sectional plan view of a spin valve sensor accordingto the invention.

FIG. 2 is a block diagram of a magnetic disk storage system according tothe invention.

FIG. 3 is a flowchart illustrating a sequence of operational steps forresetting a spin valve sensor according to the invention.

FIG. 4 is a cross-sectional perspective view of an actuator assembly inrelation to an external magnetic field according to the invention.

FIG. 5 is a diagram illustrating one exemplary waveform of electricalcurrent applied to a spin valve sensor in accordance with the invention.

FIG. 6 is a diagram illustrating a different example of electricalcurrent applied to a spin valve sensor in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hardware Components &Interconnections

The present invention concerns a technique for robustly resetting themagnetization direction of a spin valve head. This technique may beimplemented, for example, in the hardware environment described below.

Spin Valve Sensor—Materials

FIG. 1 depicts an example of a spin valve sensor 100 upon which theinvention may be practiced. The view of FIG. 1 depicts a plan view ofthe air bearing surface of a substrate 101 containing the spin valve100. The substrate's air bearing surface normally rides upon a cushionof air, which separates it from a magnetic data storage medium such as adisk or tape.

The sensor 100 includes a plurality of substantially parallel layersincluding an antiferromagnetic layer 102, a ferromagnetic pinned layer103, a conductive layer 104, and a ferromagnetic free layer 105. Thesensor 100 also includes hard bias layers 115-116, the operation ofwhich is discussed in greater detail below. The sensor 100 is depositedupon an insulator 107, which lies atop the substrate 101. Adjacentlayers preferably lie in direct atomic contact with each other.

The antiferromagnetic layer 102 comprises a type and thickness ofantiferromagnetic substance suitable for use as a pinned layer in spinvalves, e.g., a 400 Å layer of NiO. The ferromagnetic pinned layer 103comprises a type and thickness of ferromagnetic substance suitable foruse in spin valves, e.g., about 10-40 Å of Co. The conductor layer 104comprises a type and thickness of conductive substance suitable for usein spin valves, e.g., about 20-30 Å of Cu. The ferromagnetic free layer105 comprises a type and thickness of ferromagnetic substance suitablefor use as a free layer in spin valves, e.g., about 30-150 Å of NiFe.The hard bias layers 115-116 provide the free layer 105 with a desiredquiescent magnetization. The hard bias layers 115-116 preferablycomprise a magnetic material with high coercivity, such as CoPtCr.

Despite the foregoing detailed description of the sensor 100, thepresent invention may be applied using many different sensorarrangements in addition to this example. For example, ordinarilyskilled artisans having the benefit of this disclosure will recognizevarious alternatives to the specific materials and thickness describedabove.

Spin Valve Sensor—Magnetization

The sensor 100 exhibits a predefined magnetization. Magnetization of thesensor 100, including the ferromagnetic layers 103/105 and theantiferromagnetic layer 102, is performed in accordance with theinvention. The sensor 100 may be magnetized prior to initial operation,such as during the fabrication or assembly processes. Or, the sensor 100may be magnetized after some period of operating the sensor 100, wherethe sensor 100 loses its magnetic orientation due to a traumatic hightemperature event such as electrostatic discharge. A process formagnetization of the sensor 100 is discussed in greater detail below.

Whether magnetized before or after initial operation of the sensor 100,the magnetized components of the sensor 100 are ultimately given thesame magnetic configuration. In particular, the antiferromagnetic layer102 has a magnetic orientation in a direction 110. For ease ofexplanation, conventional directional shorthand is used herein, where acircled dot indicates a direction coming out of the page (like anarrow's head), and a circled “x” indicates a direction going into thepage (like an arrow's tail). The neighboring ferromagnetic pinned layer103 has a magnetic moment pinned in a parallel direction 111, due toantiferromagnetic exchange coupling between the layers 102-103.

Unlike the pinned layer 103, the free layer 105 has a magnetic momentthat freely responds to external magnetic fields, such as those from amagnetic storage medium. The free layer 105 responds to an externalmagnetic field by changing its magnetic moment, which in turn changesthe resistance of the spin valve 100. In the absence of any othermagnetic fields, the free layer 105 orients itself in a direction 113,which is oriented 90° to the directions 110-111. This quiescentmagnetization direction is due to biasing of the free layer 105 by thehard bias layers 115-116.

Electrical Current in the Spin Valve

The sensor 100 may also include various accessories to direct electricalcurrent and magnetic fields through the sensor 100. A small but constantsense current, for example, is directed through the sensor 100 toprovide a source of scattering electrons for operation of the sensor 100according to the GMR effect. At different times, a relatively largecurrent pulse or “waveform” is directed through the sensor 100 toestablish the magnetization direction of the sensor 100. FIG. 1 alsodepicts the sensor 100 in relation to the various features that helpdirect current through the sensor 100.

The sensor 100 is attached to a pair of complementary leads 108-109 tofacilitate electrical connection to a sense current source 112. Theleads 108-109 also facilitate electrical connection to a pulse currentsource 123. The leads 108-109 preferably comprise 500 Å of Ta with a 50Å underlayer of Cr, or another suitable thickness and type of conductivematerial. The attachment of leads to magnetoresistive sensors and spinvalves is a well known technique, familiar to those of ordinary skill inthe art.

Establishing Magnetization Direction

Via the leads 108-109, the pulse current source 123 directs anelectrical pulse current through the layers 103-105. Chiefly, the pulsecurrent heats the antiferromagnetic layer 102 past its blockingtemperature, as explained in greater detail below. For an additionalmeasure of magnetization biasing, the pulse current source 123 may beconfigured to provide pulse current in an appropriate direction toenhance biasing of the antiferromagnetic layer 102 in the direction 110.Using the illustrated example, pulse current for this purpose flows fromthe lead 109 to the lead 108.

To satisfy the foregoing purposes, the current source 123 comprises asuitable device to provide a current pulse of sufficient amplitude andduration to bring the antiferromagnetic layer 102 past its blockingtemperature, thereby freeing the magnetic orientations of this layer aswell as the associated ferromagnetic pinned layer 103. As an example,the current pulse may comprise a 17-18 mA signal lasting about 60 to 120nanoseconds.

It is emphasized that ordinarily skilled artisans having the benefit ofthis disclosure will recognize that certain changes may be made to thebiasing and pinning of the various sensor components without departingfrom the scope of the invention. Furthermore, more particularexplanation is provided below concerning the particular manner andeffect of biasing the sensor 100.

Sense Current

Via the leads 108-109, the current source 112 directs a small, constantelectrical current through the layers 103-105 during ongoing operationof the sensor 100. As an example, the sense current may be about 4-10 mAD.C.

Output Sensing

The sensor 100 functions because the resistance of the sensor 100changes during the detection of an external magnetic field. Othercircuitry, discussed below, quantifies this change in resistance togenerate a “playback” signal representative of the detected magneticflux transitions. The resistance of the sensor 100 is determined bymeasuring the voltage drop across occurring between the leads 108-109,and dividing this voltage by the sense current. To measure the voltagedrop, the free layer 105 may be coupled to a voltage sensor (not shown)such as a differential amplifier or another appropriate voltage sensor.Alternatively, a sense voltage may be placed across the leads 108-109,with measurement taken of the resultant current therebetween.

Magnetic Disk Storage System

FIG. 2 depicts an example of a magnetic disk storage system embodyingsensors such as the sensor 100. Ordinarily skilled artisans willrecognize, however, that invention is also applicable to other magneticrecording systems than the specific embodiment 200 illustrated in FIG.2.

A magnetic disk storage comprises at least one rotatable magnetic disk202 is supported on a spindle 204 and rotated by a disk drive motor 206with at least one slider 208 positioned on the disk 202, each slider 208supporting one or more magnetic read/write heads. The magnetic recordingmedia on each disk is in the form of an annular pattern of concentricdata tracks (not shown) on the disk 202. As the disk 202 rotates, thesliders 208 are moved radially in and out over the disk surface 210 sothat the heads 212 may access different portions of the disk wheredesired data is recorded. Each slider 208 is attached to an actuator arm214 by means of a suspension 216. The suspension 216 provides a slightspring force which biases the slider 208 against the disk surface 210.Preferably, the actuator arm 214, suspension 216, and slider 208 areembodied in an integrated suspension assembly constructed in accordancewith the invention, such as ones of the various embodiments described indetail above. Each actuator arm 214 is attached to an actuator means 218The actuator means 218 as shown in FIG. 2 may be a voice coil motor(“VCM”), for example. The VCM comprises a coil moveable within a fixedmagnetic field, the direction and velocity of the coil movements beingcontrolled by the motor current signals supplied by a controller. Duringoperation of the disk storage system, the rotation of the disk 202generates an air bearing between the slider 208 and the disk surface 210which exerts an upward force or lift on the slider. The air bearing thuscounterbalances the slight spring force of the suspension 216 andsupports the slider 208 off and slightly above the disk surface by asmall, substantially constant spacing during operation.

The various components of the disk storage system are controlled inoperation by control signals generated by control unit 220, such asaccess control signals and internal clock signals. Typically, thecontrol unit 220 comprises logic control circuits, storage means and amicroprocessor, for example. The control unit 220 generates controlsignals to control various system operations such as drive motor controlsignals on line 222 and head position and seek control signals on line224. The control signals on line 224 provide the desired currentprofiles to optimally move and position a selected slider 208 to thedesired data track on the associated disk 202. Read and write signalsare communicated to and from read/write heads 212 by means of recordingchannel 226.

The above description of a typical magnetic disk storage system, and theaccompanying illustration of FIG. 2 are for representation purposesonly. It should be apparent that disk storage systems may contain alarge number of disks and actuators, and each actuator may support anumber of sliders.

OPERATION

As mentioned above, the present invention concerns a method of robustlyresetting the magnetic orientation of a spin valve sensor using internaland external magnetic fields. As shown below, an “internal” magneticfield may be provided by a pulse of electrical current applied to thesensor 100. More importantly, an “external” magnetic field is generatedindependently of the sensor 100 and applied thereto.

Exemplary Sequence of Operation: Introduction

FIG. 3 shows a sequence of method steps 300 to illustrate one example ofthe method aspect of the invention. The sequence 300 is useful to firstestablish the magnetic configuration of one or more spin valve sensorsduring fabrication, assembly, or another stage of the manufacturingprocess. This process is also useful for resetting the magneticconfiguration of sensor(s) after the sensor(s) experience a disorientinghigh temperature event such as a transient voltage from electrostaticdischarge. FIG. 4 depicts one preferred embodiment, where a magneticfield source 400 provides an external magnetic field to reset spin valvesensors of an assembled actuator assembly 402 that includes multipleread or read/write heads 404 mounted on corresponding actuator arms 406.For ease of explanation, then, but without any limitation intendedthereby, the routine 300 is described in the context of the hardwareenvironment described above in FIGS. 1-3.

Applying External Magnetic Field

After the routine begins in task 302, the source 400 is operated in task304 to introduce an external magnetic field to the sensor 100. Thisexternal field helps overcome the limitations of previous techniquesthat relied solely upon the internal magnetic field of a current pulseto magnetically orient spin valve antiferromagnetic layers. Indeveloping the present invention, it has been discovered that themagnetic field supplied by the current-pulsing alone is not completelyuniform over the entire active area of the sensor, and also not ofsufficient magnitude. This prevents complete recovery of aelectrostatically damaged spin valve.

The present invention overcomes potential drawback by using thepreviously mentioned external magnetic field, which ensures that thesensor 100 is oriented by a uniform sufficiently and powerful field. Inthe illustrated example, this magnetic field is oriented in a directionparallel to the directions 110-111.

The magnetic source 400 preferably comprises an electromagnet or anothersource adequate to generate a magnetic field sufficient to robustlyorient the antiferromagnetic layer 102 as desired upon the applicationof a high current heating pulse to the layer 102. As an example, themagnetic field strength is preferably at least 4.5 KOe.

The external magnetic field biases each of the heads 404 in the commondirection 150. The assembly 402, however, contains up-facing heads anddown-facing heads, as defined by the direction faced by their airbearing surfaces. To accommodate this difference, the heads 404 arepreferably operated during disk operations with playback signals fromall up-facing or down-facing heads preferably inverted to match theremaining heads' polarity.

Pulse (Heating) Current

After task 304, the pulse current source 123 in task 306 applies awaveform of electrical current, having a predetermined shape, to thesensor 100. This pulse current waveform heats the antiferromagneticlayer 102 past its blocking temperature, thereby freeing the magneticorientation of this layer as well as the associated ferromagnetic pinnedlayer 103. If desired, the pulse current may be directed suitably tocooperate with the external magnetic field in aligning the magnetizationdirection of the antiferromagnetic layer 102. In this embodiment, thepulse current runs from the lead 109 to the lead 108. However, pulsecurrent with an opposite direction may be used if desired, since theinternal magnetic field provided by the pulse current is substantiallyless significant than the external magnetic field. The amplitude andduration of the current pulse are chosen to adequately bring theantiferromagnetic layer 102 past its blocking temperature. According toone embodiment, the pulse current waveform may comprise a square wavepulse 500 as shown in FIG. 5. As an example, the pulse 500 may last forabout 50-150 nanoseconds, and have an amplitude of about 17-18 mA.

Experiments show, however, that the current pulse polarity and its widthare non-critical when used in conjunction with the external field. Thisis due to the fact that the purpose of the current pulse is to raise thelocal temperature while the actual setting of the pinned layermagnetization is accomplished by the external field.

According to a different embodiment of the invention, the pulse currentwaveform may instead comprise a multi-tiered square wave pulse havingdifferent amplitudes at different times. One example is the pulse 600appearing in FIG. 6. The pulse 600 includes an increased-current portion602 followed by a reduced-current portion 604. After theincreased-current portion 602 heats the antiferromagnetic layer 102beyond its blocking temperature, the reduced-current portion 604cooperates with the external magnetic field to establish the desiredmagnetic orientation of the antiferromagnetic layer 102. The use of thereduced-current portion 604 also saves energy and helps avoid damage tothe sensor 100.

In addition to the waveform shapes 500-600, ordinarily skilled artisanshaving the benefit of this disclosure will recognize that many otherwaveform shapes can be utilized without departing from the scope of thepresent invention.

After completion of the pulse current, the antiferromagnetic layer 102cools below its blocking temperature, fixing its magnetic orientation inthe direction 110. The ferromagnetic pinned layer 103 is oriented in aparallel direction 111 due to its high exchange coupling with theantiferromagnetic layer 102.

Applying Second Magnetic Field

Following application of the current source in task 306, task 308removes the external magnetic field. Next, in task 310 a differentexternal magnetic field is applied to the sensor 100 to orient the hardbias layers 115-116 in the direction 113. This field may be generated bythe magnetic field source 400, for example, or a different source ifdesired. As an example, the field strength is preferably at least 4.5KOe, and lasts for a sufficient time to orient the hard bias layers 115and 116 with the applied magnetic field. Coupling between the hard biaslayers 115 and 116 and the free layer 105 biases the free layer in thedirection 113 during its quiescent state.

After task 310, the magnetic field from task 310 is removed, and thenthe routine 300 ends in task 314.

OTHER EMBODIMENTS

While there have been shown what are presently considered to bepreferred embodiments of the invention, it will be apparent to thoseskilled in the art that various changes and modifications can be madeherein without departing from the scope of the invention as defined bythe appended claims. For example the specifically disclosed directionsof magnetization orientations, magnetic fields, and the like may bereversed. Thus, the response of the sensor 100 to external magneticfields would be opposite that shown.

What is claimed is:
 1. A method for establishing magnetic orientationsin first and second directions of an array of read heads in a head diskassembly, one group of read heads in the array facing a first directionand another group of read heads in the array facing a second directionsubstantially opposite the first direction, each read head including aspin valve sensor which has a pinning layer, the method comprising:applying a first external magnetic field to the array of read heads insaid first direction; during application of the external magnetic field,applying a pulse of electrical current to each spin valve sensor withsufficient amplitude and duration to permit magnetic spins of pinninglayer in the spin valve sensors to rotate because of heat generated bysaid pulse of electrical current; ending said pulse of electricalcurrent; after ending said pulse of electrical current, removing thefirst external magnetic field; and applying a second external magneticfield to the array of read heads in said second direction for apredetermined time and then removing the second external magnetic field.2. A method for establishing magnetic orientations in first and seconddirections of an array of read heads in a head disk assembly, one groupof read heads in the array facing a direction opposite another group ofread heads in the array, each read head including a spin valve sensor,comprising: each spin valve sensor including: a ferromagnetic pinnedlayer; an antiferromagnetic pinning layer exchange coupled to the pinnedlayer for pinning a magnetic moment of the pinned layer in said firstdirection; a ferromagnetic free layer having a magnetic moment which canbe oriented in said second direction when a sense current is conductedthrough the sensor and which is free to rotate from the second directionin response to signal fields from a moving magnetic medium; anonmagnetic conductive spacer layer located between the pinned and freelayers; and a hard bias layer adjacent the free layer for biasing themagnetic moment of the free layer in said second direction; the methodcomprising the steps of: applying a first external magnetic field to thespin valve sensors in said first direction; during application of thefirst external magnetic field, directing a pulse of electrical currentthrough each spin valve sensor with sufficient amplitude and duration topermit magnetic spins of each pinning layer to rotate in response tosaid first external magnetic field; terminating said pulse of electricalcurrent and said first external magnetic field; after said terminating,applying a second external magnetic field to each spin valve sensor insaid second direction; and terminating the second external magneticfield after a predetermined period of time.
 3. The method of claim 2wherein the first external magnetic field is applied prior to directingthe pulse of electrical current through each spin valve sensor.
 4. Themethod of claim 2 wherein the second orientation direction issubstantially perpendicular to the first direction.
 5. The method ofclaim 2 wherein the pulse of electrical current is a waveform ofelectrical current having multiple different levels of amplitude.
 6. Themethod of claim 2 wherein the pulse of electrical current has a squarewave shape.
 7. The method of claim 2 wherein the pulse of electricalcurrent has a magnitude of about 14-22 mA.
 8. The method of claim 7wherein the pulse of electrical current has a duration of about 50-150nanoseconds.
 9. The method of claim 2 wherein the second externalmagnetic field has a strength equivalent to a strength of the firstexternal magnetic field.
 10. The method of claim 9 wherein the secondexternal magnetic field has a field strength of about 4.5 KOe.
 11. Themethod of claim 2 wherein the first external magnetic field hassufficient strength to orient the magnetic moment of each pinned layerin said first direction.
 12. The method of claim 11 wherein the firstexternal magnetic field is applied prior to directing the pulse ofelectrical current through each spin valve sensor.
 13. The method ofclaim 12 wherein the second orientation direction is substantiallyperpendicular to the first direction.
 14. The method of claim 13 whereinthe pulse of electrical current is a waveform of electrical currenthaving multiple different levels of amplitude.
 15. The method of claim14 wherein the pulse of electrical current has a square wave shape. 16.The method of claim 15 wherein the pulse of electrical current has amagnitude of about 14-22 mA.
 17. The method of claim 16 wherein thepulse of electrical current has a duration of about 50-150 nanoseconds.18. The method of claim 17 wherein the second external magnetic fieldhas a strength equivalent to a strength of the first external magneticfield.
 19. The method of claim 18 wherein the second external magneticfield has a field strength of about 4.5 KOe.